charging the bootstrap cap

Lots of half-bridge and full-bridge mosfet gate drivers use a
bootstrap capacitor to make the necessary gate drive for the upper
n-fet. The upper-fet gate needs to swing maybe 10 volts above the
drain supply, which is generally the biggest voltage around.

But the first time the driver is enabled, the boot cap may not be
charged. And it will eventually discharge, given a long enough
positive output pulse. I've never seen a data sheet mention this.

I think I recall some driver that has a charge pump, but that's rare.

Halfway driving a fet is a good way to blow it up. I haven't done that
this week, but it is a concern.